Computer disk drive systems, such as hard disk drives, CD-ROM drives, and floppy disk drives, are widely used to store information for use by computers that include the disk drive systems. A typical disk drive system 10 is schematically shown in FIG. 1. The disk drive system 10 includes a digital control unit 12, such as a central processing unit (CPU), and a motor control circuit 14 that together control a motor 16, typically referred to as a voice coil motor.
The voice coil motor 16 typically includes a single coil situated inside a tubular permanent magnet and is connected to the motor control circuit 14 by a plurality of flex cables 18. The coil is free to move inside the magnet subject to a minimal amount of damping and friction from the flex cables 18. An actuator arm 20 is connected between the coil and a read/write head 22 that is positioned on a rotatable storage disk 24. The actuator arm 20 and read/write head 22 move backwards and forwards in a linear path along with the coil.
The coil, actuator arm 20, and read/write head 22 are moved in a forward direction by driving current through the coil in a forward direction and are moved in a backward direction by driving current through the coil in a backward direction. The speed at which the coil, actuator arm 20, and read/write head 22 are moved depends on the amount of current driven through the coil. The coil, actuator arm 20, and read/write head 22 will maintain momentum in the forward direction until a current is driven through the coil in a backward direction resulting in a negative acceleration. The coil, actuator arm 20, and read/write head 22 will slow and stop according to the amount of current driven through the coil in the backward direction, and a further application of current will accelerate the coil in the backward direction.
The storage disk 24 typically has a magnetic surface, or an optically readable surface for CD-ROM drives, with a plurality of concentric tracks in which digital data is stored. The read/write head 22 is positioned by the motor 16 over a track to read data in the track or to write data to the track. The data written to and read from the storage disk 24 are conveyed between the CPU 12 and the read/write head 22 by a data bus 26.
The position of the read/write head 22 is determined by a control logic unit in the disk drive system from two sources of information. The read/write head 22 itself reads position data from the rotating storage disk as it travels over the storage disk to the selected track and transmits this data to the CPU 12 over the data bus 26. In addition, the CPU 12 retains in a memory preassigned velocity profiles to achieve a desired track position. The CPU 12 senses the track position and the amount of current in the coil to determine the acceleration of the coil, and then compares this information with the velocity profiles stored in the memory to optimize the position of the read/write head 22.
The motor 16 needs a relatively high voltage supply (e.g., 12 V) to drive the coil, actuator arm 20, and read/write head 24 back and forth to position the read/write head over the desired tracks. However, the digital drive commands issued by the CPU 12 to position the read/write head 24 are of a relatively low voltage level (e.g., 5 V or less). That is, the digital drive command typically are expressed in binary bits of logic level 1 or 0, with a I logic level corresponding to 5 V and a 0 logic level corresponding to 0 V.
To convert the low voltage digital command signals from the CPU 12 to the high voltage analog driving signals needed to drive the motor 16, the motor control circuit includes a digital to analog converter (DAC) 28 and a level shifter 30. As its name implies, the DAC 28 converts the digital command signals to low voltage analog command signals. The level shifter 30 converts the low voltage analog command signals to the high voltage analog driving signals needed to drive the motor 16. The level shifter 30 supplies the high voltage analog driving signals to a voice coil motor (VCM) driver 32 which drives current through the motor coil to move the read/write head 22 appropriately. Often the level shifter 30 is part of the VCM driver 32 and the VCM driver 32 may include more than one level shifter.
A problem with prior art disk drive systems, such as the disk drive system 10, is that the level shifter 30 (or plural level shifters) degrades system performance in various ways. For example, each level shifter 30 causes unwanted signal offset, that is, the output of each level shifter is finite even with zero input. Moreover, each level shifter 30 reduces the power supply rejection ratio of the motor control circuit 14, that is, the error added to the analog command signals because of power supply fluctuations. Such errors introduced by each level shifter 30 degrade the fine control of the analog command signals provided by the DAC 28.